1. Field of the Invention
The present invention relates to a miniaturized voice coil motor apparatus with a magnetically pre-loaded precision guidance, particularly to a voice coil motor apparatus, which drives the lens module of miniaturized camera.
2. Description of the Related Art
As shown in FIG. 5, the conventional cylindrical voice coil motor comprises: a concentric yoke 51 having a U-shape section; multiple segments of arc-type magnets 52, which is attached to the inner face of the U-shape sectional concentric yoke 51 to form a closed magnetic field; and a coil 53, disposed inside the gap 54 between the magnet 52 and the yoke 51; wherein a current flowing through the coil 53 will interact with the magnetic field inside the gap 54, which will generate a thrust. In order to maintain the uniformity of the spacing between the coil 53 and the magnet 52 and the spacing between the coil 53 and the yoke 51, several methods can be used to support and guide the coil 53 inside the cylindrical voice coil motor: firstly, a precision cylindrical linear bearing is adopted, wherein the cylinder bushing portion is rigidly coupled to the yoke 51, and the coil 53 is rigidly coupled to the spool portion, and then a non-contact precision and parallel sliding motion between the coil 53 and the yoke 51 can be maintained as the precision cylindrical linear bearing has rolling balls installed there between; secondly, when the coil 53 moves only a relatively short distance, two pieces of elastic platelets are rigidly coupled to the coil 53, which can also achieve the non-contact precision and parallel sliding motion between the coil 53 and the yoke 51 theoretically if both elastic platelets have an exactly identical elastic modulus; thirdly, in the non-precision application, the coil 53 is coupled to a sliding cylinder, wherein the inner rim of the cylinder slidably contacts the outer rim of the yoke 51 in order to achieve a non-contact sliding motion between the coil 53 and the yoke 51, and wherein the friction and parallelism of motion depends on the gap resulting from the tolerances of the inner and the outer rims.
However, the application of the conventional voice coil motor to drive the lens module of the miniaturized camera has the following bottlenecks:    (a) The lens is a movable piece and driven by the voice coil motor to move back and forth to achieve the objective of auto-focus or variable-focus, which, in addition to a high repeatability of positioning, also demands a high standard about the runout between the motion axis and the optical axis. If the first method mentioned above is adopted, the accomplishments in the motion repeatability and the friction are superior, but the precision linear bearing is expensive and has a bottleneck in miniaturization. In the antecedent of mass-producing the miniaturized camera, it is not practical. If the second method mentioned above is adopted, the image quality of the miniaturized camera will be influenced owing to the runout resulting from the difference in the elastic moduli of the elastic platelets or the difference in the magnetizations of multiple segments of arc type magnets; further, in its application to the portable miniaturized camera, the gravity of lens will also results in motion runout; therefore, adopting the second method will incur a lower yield in mass-production; moreover, the gravity-induced runout will obviously worsen the image quality in the future high-pixel miniaturized camera. If the third method mentioned above is adopted, the runout of the motion axis is controlled via the confinement of the strict assemblage tolerance between the moving and the non-moving members; however, the precision of the component injection molds are not so uniform and hard to control, which will result in unstable quality in mass-production, wherein the components are assembled randomly and the friction between the moving and non-moving members and the angle of motion runout are variable; too tight tolerance induces too high friction, which further induces the voice coil motor to be hard to drive as the force output by the voice coil motor is so limitary; too loose tolerance induces too large free-play angle, which further induces the image quality to be lowered down. Therefore, it is desired to invent a low-cost guiding mechanism, wherein the friction and the runout angle are irrespective of the assemblage tolerance between the moving and non-moving members and the gravity of the lens module, to enable the mass-production of voice coil motor, which is applied to the lens module of the miniaturized camera, to be realized.    (b) In order to satisfy the demand for the portable miniaturized camera, miniaturizing the modules' size to meet the dimensional limitation in the portable device is the current tendency in the market; however, the dimensional miniaturization is limited by the fabrication capability of the thin magnet owing to the symmetric structure of the conventional voice coil motor; therefore, how to break through the above problems from the view of magnetic structure design to achieve the objective of size miniaturization is an important subject.